IB or A Levels?

For medicine, vet med, and dentistry IB is quite good, as you satisfy the scientific criteria (providing you take chem/bio as a second science for your group 6 subject). You are also are necessarily required to do a reasonable amount of maths (i.e. through calculus) which is a useful contextual skill, although you won't be actively using it much, and the language work may be helpful, although this is debatable.

You will also however have to do a good deal of writing in your group 1 and 3 options, which is a very useful skill since as I understand there is more writing on these courses than comparable science courses (a friend of my sister apparently was saying how she had to teach basic essay writing technique to her colleagues at Cambridge since they all did triple/quad sciences and hadn't written essays since GCSE).

For bioscience degrees (i.e. not professional medical and related courses) it's OK, but you may be better served by A-levels where you can maximise your science options, and focus entirely on these alone. While IB is suitable for this as well, you're spread a bit more thin and unlike medicine and related where the signifcantly broad workload benefits from this background, for the more specific and arguably less intensive in that sense work of an average science degree it's a bit overkill at best.

Either is suitable for both overall, but IB is maybe slightly better preparation for the variety and volume of work in medical type courses and A-levels perhaps slightly more useful for being more focused on the core sciences necessary. In terms of syllabus of the two sciences they are mostly the same. IB Chemistry has been reputed by some to be a bit harder than A-level Chemistry as it goes a bit more in depth into some areas apparently, and IB Biology conversely slightly less so as it covers slightly less breadth of material than A-level.

IB also has an option paper for both subjects, where you can study a variety of options outside the usual syllabus for this level such as neurobiology and behaviour or evolution for biology, of for chemistry examples such as energy or medicinal chemistry. The options are normally determined by your teacher, although all options are present on the options paper so you can potentially self study a different one.